1. Field of the Invention
The present invention relates to a potential controlling method and potential controller of an image forming apparatus, such as a copying machine, a printer, or a facsimile, using an electrophotographic system, and particularly to a potential controlling method and potential controller of an image forming apparatus, which can certainly prevent a picture quality defect or the like caused by adhesion of toner or carrier to the surface of an image bearing body at the time of rising or falling of a charged potential of the image bearing body and a developing bias voltage.
2. Description of the Related Art
Conventionally, an image forming apparatus, such as a copying machine, a printer, or a facsimile, using the electrophotographic system is constructed such that after the surface of a photoreceptor drum is charged to a predetermined potential, the image exposure of the surface of the photoreceptor drum is carried out to form an electrostatic latent image, and the electrostatic latent image is developed by a developing unit, so that a desired image is formed.
In the image forming apparatus like this, at the time of start of charging of the photoreceptor drum, as shown in FIG. 9A, when a non-charged region of the surface of the photoreceptor drum which is not charged is moved to a development position facing a developing roll of the developing unit as the photoreceptor drum is rotated, and when a developing bias voltage Vdc is applied to the developing roll, an electric field in the direction to move toner from the developing roll to the surface of the photoreceptor drum is formed between the photoreceptor drum and the developing roll by a potential difference Vimg between the developing bias voltage Vdc and the potential of the non-charged region of the photoreceptor drum, that is, 0 V. Thus, the toner or the toner and carrier in a developer adhere to the surface of the photoreceptor drum.
Besides, in the above image forming apparatus, at the time of start of charging of the photoreceptor drum, as shown in FIG. 9C, when a charged region of the surface of the photoreceptor drum is moved to the development position facing the developing roll of the developing unit as the photoreceptor drum is rotated, and when the developing bias voltage Vdc of the developing roll remains OFF, an electric field in the direction to move the carrier charged with reverse polarity of the toner from the developing roll to the surface of the photoreceptor drum is formed between the photoreceptor drum and the developing roll by a potential difference between a charged potential Vh of the photoreceptor drum and 0 V of an OFF potential of the developing bias voltage. Thus, the carrier immediately adheres to the charged region of the surface of the photoreceptor drum by the above electric field.
The adhesion phenomenon of the toner and the carrier occurs not only at the time of start of charging of the photoreceptor drum as the time of start of image formation, but also at the time of stop of charging of the photoreceptor drum as the time of end of image formation with the same reason as the above.
Like this, when the toner and the carrier adhere to the surface of the photoreceptor drum, in the image forming apparatus using an intermediate transfer belt, there have been problems that the photoreceptor drum, the intermediate transfer belt or the like is fouled or damaged, and in the region where the carrier adheres to the surface of the photoreceptor drum, transfer of a toner image is not excellently carried out, so that a defect in picture quality, such as a white point or white streak on a printed sheet, is caused.
As a technique which can solve the problems caused from the adhesion of the toner and the carrier to the surface of the photoreceptor drum at the time of start or stop of image formation, there has been already proposed a technique disclosed in Japanese Patent Unexamined Publication No. Hei. 9-329946 or No. Hei. 7-253693.
An image forming apparatus of the foregoing Japanese Patent Unexamined Publication No. Hei. 9-329946 includes a charging part including a charging member for uniformly charging the surface of an image bearing body and a charging power source for applying a voltage to the charging member, an exposure part for attenuating the potential of the surface of the image bearing body charged by the charging part with image exposure to form an electrostatic latent image on the image bearing body, and a developing part including a developer bearing body for bearing a two-component developer containing a toner charged in the same polarity as the charging polarity of the charging part and a carrier charged in the reverse polarity to the charging polarity, and a developing bias power source for applying a developing bias voltage of the same polarity as the charging polarity to the developer bearing body, and the image forming apparatus further includes a control part for controlling the turning ON/OFF of the charging power source and the developing bias voltage power source so that application of the developing bias voltage to the developer bearing body starts when the end portion of charging start of the surface of the image bearing body faces the developer bearing body, and the application of the developing bias voltage is stopped when the end portion of charging stop of the surface of the image bearing body faces the developer bearing body, and the ON/OFF timing of the charging power source and the developing bias power source is set so as to make the value of the potential of the surface of the image bearing body approach the developing bias voltage under the condition that the absolute value of the potential of the surface of the image bearing body, which passes through the facing position to the developer bearing body, becomes equal to or less than the absolute value of the developing bias voltage when the developing bias voltage is transiently changed by ON/OFF of the developing bias power source.
A potential control method in an image forming apparatus of Japanese Patent Unexamined Publication No. Hei. 7-253693 uses a two-component developer made of a toner and a carrier, and in an image forming apparatus in which the toner is made to adhere to a photoreceptor by applying a developing bias voltage to a developing sleeve, both the surface potential of the photoreceptor and the developing bias are controlled in stages at the time of start and stop of the rotation of the photoreceptor.
However, the foregoing prior art has problems as follows. That is, in the case of the foregoing image forming apparatus of Japanese Patent Unexamined Publication No. Hei. 9-329946, the ON/OFF timing of the charging power source and the developing bias power source is set so as to make the value of the potential of the surface of the image bearing body approach the developing bias voltage under the condition that the absolute value of the potential of the surface of the image bearing body, which passes through the facing position to the developer bearing body, becomes equal to or less than the absolute value of the developing bias voltage when the developing bias voltage is transiently changed by ON/OFF of the developing bias power source. Thus, as shown in FIG. 9B, if the timing of the charged potential of the image bearing body is coincident with that of the developing bias voltage at the developing position, any problem does not occur. However, the charging power source and the developing bias power source have difference and fluctuation in rising/falling speed, and the image bearing body also has rotation variation or the like. Thus, there have been problems that it is difficult to make the timing of the charged potential of the image bearing body coincident with that of the developing bias voltage at the developing position, and it is difficult to certainly prevent the toner and carrier from adhering to the surface of the image bearing body at the time of start and stop of the image formation. The problems become more remarkable when the rotation speed of the image bearing body becomes high and the process speed becomes high.
In the case of the potential control method in the image forming apparatus of Japanese Patent Unexamined Publication No. Hei. 7-253693, both the surface potential of the photoreceptor and the developing bias are controlled in stages at the time of start and stop of the rotation of the photoreceptor. However, with the same reason as the technique of Japanese Patent Unexamined Publication No. Hei. 9-329946, there has been a problem that it is difficult to certainly perform the control in stages of both the surface potential of the photoreceptor and the developing bias at the time of start and stop of the rotation of the photoreceptor. Further, in the case of the potential control method in the image forming apparatus of Japanese Patent Unexamined Publication No. Hei. 7-253693, since both the surface potential of the photoreceptor and the developing bias are controlled in stages, in view of the difference and fluctuation in the rising/falling speed of the charging power source and the developing bias power source, the rotation variation of the photoreceptor, and the like, there has been a problem that the control in stages of both the surface potential of the photoreceptor and the developing bias takes a long time, and it is hard to apply the method to an image forming apparatus having a high printing speed.
The present invention has been made in view of the above circumstances and provides a potential controlling method and potential controller of an image forming apparatus, which can certainly prevent toner and carrier from adhering to the surface of an image bearing body at the time of start and stop of image formation, and can also be applied to an image forming apparatus having a high image forming speed.
According to an aspect of the present invention, in an image forming apparatus which forms an image by charging an image bearing body with a charging part, by performing image exposure to the surface of the image bearing body to form an electrostatic latent image, and by applying a developing bias voltage to a developer bearing body of a developing part to develop the electrostatic latent image, a potential controller of the image forming apparatus is constructed such that at the time of rising or falling of a charged potential of the image bearing body and a developing bias voltage, at least one of the charged potential of the image bearing body and the developing bias voltage is controlled to an objective value through plural stages with the rising or falling of the other occurring through the stages, so that a potential difference between a potential of a non-charged portion of the image bearing body and the developing bias voltage does not exceed a first predetermined value, and a potential difference between a potential of a charged portion of the image bearing body and the developing bias voltage does not exceed a second predetermined value.
At the time of the rising or falling of the charged potential of the image bearing body and the developing bias voltage, at least one of the charged potential of the image bearing body and the developing bias voltage is controlled as described above. However, more preferably, at both the rising and falling of the charged potential of the image bearing body and the developing bias voltage, at least one of the charged potential of the image bearing body and the developing bias voltage is controlled as described above. Thus, at least one of the time of the rising and the time of the falling of the charged potential of the image bearing body and the developing bias voltage, at least one of the charged potential of the image bearing body and the developing bias voltage is controlled as described above.
According to another aspect of the present invention, in the above potential controller of the image forming apparatus, a potential controlling method controls, at the time of rising or falling of a charged potential of the image bearing body and a developing bias voltage, at least one of the charged potential of the image bearing body and the developing bias voltage to reach an objective value through plural stages with the rising or falling of the other occurring through the stages, so that a potential difference between a potential of a non-charged portion of the image bearing body and the developing bias voltage does not exceed a first predetermined value, and a potential difference between a potential of a charged portion of the image bearing body and the developing bias voltage does not exceed a second predetermined value.